1. Field of the Invention
The present invention relates to an electroacoustic transducer used in a buzzer etc. for converting an electric signal supplied thereto into a sound.
2. Description of the Related Art
A conventional electroacoustic transducer is provided with a housing 102 formed of synthetic resin as illustrated in FIG. 5. The housing 102 is dividedly composed of a casing body 102A and a casing cap 102B. The casing body 102A is a cylindrical body having openings at both ends thereof while the casing cap 102B is a short cylindrical body which is closed at the ceiling surface thereof. Such a construction is required by the inner structure of the electroacoustic transducer and the facility of assembly thereof.
The casing body 102A accommodates a magnetic driving portion 104 as a means for converting an electric signal into an oscillating magnetic field. The magnetic driving portion 104 comprises a magnetic core 106 having a pillar shape at the central portion thereof and a coil 108 wound around the magnetic core 106. The coil 108 is energized by an alternating current supplied through an input terminal by an external device. The magnetic core 106 is surrounded by an annular magnet 110 which generates a bias magnetic field.
A stepped supporting portion 112 is formed on the inner side of the upper opening portion of the casing body 102A to support a diaphragm 114. Since the diaphragm 114 is composed of a thin elastic magnetic member which is attracted and magnetized by the annular magnet 110, the circumferential edge of the diaphragm 114 is held by the stepped supporting portion 112 and a closed magnetic path is formed between the magnetic core 106 and the annular magnet 110. A magnetic piece 116 is attached to the central portion of the diaphragm 114 to add mass thereto and increase magnetic flux density.
The casing body 102A is closed by the casing cap 102B at the side of the upper opening portion thereof to form a resonance chamber 118 serving as a resonance space and a sound emitting cylinder 120 is formed in the ceiling portion of the casing cap 102B to allow the resonance chamber 118 to be open to the atmosphere. A sound emitting hole 122 in the sound emitting cylinder 120 is a means for emitting the resonant vibration of the resonance chamber 118 to the atmosphere.
As to assembling the housing 102 of such an electroacoustic transducer, a stepped portion 124 is provided at the lower portion of the casing cap 102B to form a small diameter portion 126 which is smaller in outer diameter than that of the casing cap 102B with the casing cap 102B being designed to have the same outer diameter as that of the casing body 102A. The small diameter portion 126 is inserted into the casing body 102A until the stepped portion 124 is brought into close contact with the edge surface of the casing body 102A. The stepped portion 124 is indispensable for high sealing performance of the structure in joining the casing cap 102B to the casing body 102A by way of ultrasonic welding etc. A minute gap 128 is formed between the end surface of the opening portion of the small diameter portion 126 of the casing cap 102B and the stepped supporting portion 112 of the casing body 102A to restrain the edge of the diaphragm 114 therein. That is, the diaphragm 114 is restrained in the upper limit of movement by the casing cap 102B so as to be prevented from getting out of position or being deformed even if a strong shock is given thereto.
In such an electroacoustic transducer as set forth above, the capacity of the resonance chamber 118 is a main factor for realizing a necessary sound characteristic. Moreover, it is necessary to divide the housing 102 into the casing body 102A and the casing cap 102B for the inner structure of the electroacoustic transducer or for the facility of assembly thereof. And in case the casing body 102A and the casing cap 102B are integrated into the housing 102, it is necessary to take a measure to improve the external accuracy of the housing 102 and the positional accuracy of the diaphragm 114. A small-sized electroacoustic transducer is usually about 10 mm in diameter, and when the casing body 102A and casing cap 102B are joined each other poorly in accuracy, i.e., there occurs a discrepancy therebetween, it causes the deterioration of external appearance of the housing 102 and in some cases the product becomes unqualified merchandise.
Accordingly, in a conventional electroacoustic transducer, the casing cap 102B is made thick so as to facilitate positioning the same relative to the casing body 102A and prevent the diaphragm 114 from getting out of position. Such a structure, however, reduces the volume ratio of the resonance chamber 118 to the whole capacity or volume of the housing 102, so that the sound characteristic is deteriorated and the miniaturization or flattening of the housing 102 and consequently the miniaturization of the electroacoustic transducer is prevented.